Coaxial bridge



A. ALFORD COAXIAL BRIDGE Oct. 30, 1956 Filed July 25 1950 2 Sheets-Sheetl INVENTOR. Afldrew Al /5rd A. ALFORD COAXIAL BRIDGE Oct. 30, 1956 2Sheets-Sheet 2 Filed July 25, 1950 INVENTOR. Aha raw A/filrd UnitedStates Patent COAXIAL BRIDGE Andrew Alford, Cambridge, Mass. ApplicationJuly 25 1950, Serial No. 175,694

8 Claims. (Cl. 33311) The present invention relates to what may becalled a coaxial bridge which may be more conveniently used as a circuitarrangement than the known transmission line bridge which becomescomplex and relatively expensive to manufacture when used in connectionwith coaxial lines for feeders and loads.

The coaxial bridge circuit of the present invention is a simpler andbetter arrangement than other types of feeder systems and provides thesame or more freedom in circuit operation. In accordance with thepresent invention the coaxial bridge may provide two input ends and twooutput ends and when the loads in the input ends are balanced, no energyfrom one input end will be delivered at the opposite output end. Theinput from one end feeds the two loads in the same phase, while theinput from the opposite ends feeds the loads in opposite phase.

The coaxial bridge of the present invention is applicable to many typesof feeder systems and is particularly applicable to a localizer antennasystem used for instrument landing where the main array is intended toproduce the principal localizer course along the runway and thesubsidiary array is used to provide the background course. The presentinvention may also be used as a measuring bridge since an unknownimpedance may be connected at one load terminal and this will be exactlybalanced when no energy appears at the opposite input end from where theenergy is impressed. When used for an array with principal localizer forthe runway and subsidiary array for the background course, the load mustbe fed in the same phase for one type of pattern and in the oppositephase for the other type of pattern for energizing this system. Thepresent invention of a coaxial bridge circuit readily provides operationof this system.

Further advantages and features of the present invention will be morereadily appreciated from the description in the specification set forthbelow when taken in connection with the drawings illustrating anembodiment of the same, in which:

Figure 1 shows a longitudinal section through the coaxial bridgeassembly of the present invention.

Figure 2 shows a cross section taken substantially along the line 22 ofFigure 1.

Figure 3 shows a cross section taken on the line 3-3 of Figure 1.

Figure 4 shows a cross section taken along the line 4-4 of Figure 1.

Figure 5 shows a cross section taken along the line 5--5 of Figure l,and

Figure 6 shows a cross section taken along the line 66 of Figure 1.

In the arrangement indicated in the drawings, the coaxial bridgecomprises two input ends A and B and two load ends I and II. The inputend A comprises a terminal 10 of an inner conductor surrounded by acoaxial tube 11 which comprises the outer conductor. The inner conductor10 at the end A may be supported by an insulating collar 12 spacing itfrom the outer conductor. The outer conductor 11 is threaded into ashoulder 13 which has an outwardly tapering wall 14 fitting into anouter cylinder conductor 15 to which it is rigidly joined or integraltherewith. The inner conductor 10 within the conical wall 14 is alsoflared outward forming a conical end 16 which is joined at the end of atube 17 and made an integral element with it. The tubes 15 and 17, theconical members 14 and 16 and the inner conductor 10 and outerconducting element 11 are all coaxially positioned. The tube .17 isprovided with two diametrically opposed slots 3 and 4 extendingsubstantially the length of the tube 17 but not into the very end of thetube so that the slots are completely surrounded by metal. At the inputend B the outer conductor comprises the threaded collar or tube 18,while the inner conductor 5 coaxially positioned with it is insulatedtherefrom by a suitable insulating ring 20. The member 18 is threadedsecurely or secured in some other way to an end plate 21 which isconductive and which holds securely the end of the tube 15 in a shoulder22 forming a good conductive joint. The tube 17 is also securely held ina shoulder or recess in the plate 21 in a good conductive joint.

All of the elements in the present invention are conductive with theexception of the insulating members used as spacers or insulatorsbetween the various inner or outer conductors as indicated by thesectioning. It should be noted that the slots 3 and 4 terminate justshort of the upper end of the tube 17 so that the end of the tube 17fitting into the recess 23 in the plate 21 provides a secure shortcircuit at the end of the slots 3 and 4. The slots 3 and 4 are alsoshort circuited by the conical elements 16 at the left end of the tube17. Other suitable methods of short circuiting the ends of the slots mayof course be used. At the upper end of the coaxial bridge as shown inFigure 1, the inner conductor 5 which is preferably positioned coaxiallywith the tubes 17 and 15 extends substantially to a mid-position of thelength of the slot and is then turned at a right angle and connected tothe wall of the tube 17 in a position substantially mid-Way between theslots 3 and 4 as indicated at 19. From this point of connection aconductor 24 extends outward through a hole in the tube 15 forming theinner conductor for the load I. The outer conductor for the load I isformed by the element 25 which is conductively united with the outerwall of the tube 15. The element 25 acts as the outer conductor for theinner conductor 24 and forms the coaxial load terminal for the load I.An insulating ring 26 spaces and supports the inner conductor 24 fromthe outer conductor 25. Extending from a diametrically opposite positionwith reference to the right angle bend in the conductor 5, a conductiveconnection 27 is made to the wall of the tube 17 in a position mid-waybetween the slots 3 and 4. This conductor 27 extends outward through ahole in the tube 15, is insulated from it and forms the inner conductorfor the load II. The outer conductor for the load ll is formed by theelement 28 which surrounds and is coaxial with the conductor 27 and isconductively secured to the outer wall of the tube 15.

When current is fed into the input end A, the coaxial feed comprises thetube 17 forming the inner conductor and the tube 15 forming the outerconductor. Therefore the load at I is energized with the outer conductor25 connected to the outer conductor 15 and the inner conductor 24connected to the inner conductor 17. The same phase of energization isalso provided for the load II since the outer conductor 28 is connectedto the outer conductor 15 and the inner conductor 27 is connected to theinner conductor 17. The outputs of the loads are therefore in the samephase with each other. From the symmetry of the arrangements and theconnections, it will be seen that no power comes out of B when the loadsI and II are the same. Both outer and inner conductors are at the end Bunder these conditions effectively short-circuited by the conductive endplate 21. The distance between the end plate 21 and the mid-position ofthe slot where the inner conductors of the loads I and I] are connectedshould not be a half wave length or a whole multiple of a half wavelength. This distance is preferably but not necessarily made equal to M4where M4 is the average operating frequency or a frequency within theoperating range.

When power is fed into the end B with equal loads at I and II, equal andopposite potentials are set up across the gaps in the slots 3 and 4 withthe potential at the ends of the slots equal to zero because ofeffective short circuit by the conduction material around the end of thegap. The conductors 24 and 27 assume equal and opposite potentials andthe outer conductors 25 and 23 equal and opposite potentials from thatof the conductors 24 and 27. It is diflicult if not impossible toprovide an accurate corresponding circuit diagram but the physicalanalysis appears to be plain.

Assume the presence of two charges of positive potential travelling downthe conductor 5 and two correspondnegative charges travelling down theconductor 17 each attracting their opposites. The slots 3 and 4 will, ifsymmetrically placed, distribute half the negative charge in the part ofthe tube 17 on the right of conductor 5 and half in the tube 17 on theleft of conductor 5. At the mid-connecting point where the conductor 5connects to the inner side of tube 17, the charge there on tube 17becomes neutralized but only to the extent of half or a portion of thecharge on conductor 5. In fact at the connecting point 19 the course ofthe charge can be in three directions; down the tube 17, up the tube 17and outward through the conductor 24. Where the system is balanced andthe loads are equal and where the lengths from the point 19 to the endsof the slot are substantially an effective one quarter of a wave lengthof the operating frequency of propagation all the remaining charge mayfiow out through the conductor 24. Since the other negative charge onthe opposite (right referred to Figure l) half of the tube 17 isreleased at the junction of the conductor 5 to the tube 17, this chargeflows out through conductor 27.

The corresponding opposite charges are built up on the tube to offsetthe charges on the inner conductors coaxial cable terminals of leads Iand II. For the power fed in at the end B the two halves of the tube 17below the bend where the junction 19 is made form a shunt impedancebetween the conductors 19 and 27. It will be noted also that the tube 15even though conductive has built up in it opposite potentials at theouter conductor elements and 28.

When the coaxial bridge is used as a measuring circuit, an unknownimpedance may be placed at load I and power may be impressed at A or B.A known impedance is adjusted at load II until the power opposite to theinput end is zero. The loads then will be balanced.

While the embodiment shown in the drawings show cylindrical elements, itis to be understood that other cross sectional forms may be used for thecoaxial conductors without departing from the scope of the inventionproviding the effects and results as herein may be obtained.

Having now described my invention, I claim:

1. A four terminal network comprising four coaxial transmission lines,the outer conductors of the four lines being electrically connected to acylindrical shell forming a cylindrical cavity, the inner conductor ofthe first coaxial line entering through one end of the cylindricalcavity and divided into two substantially spaced parallel parts of thesame conductor for the length of the cavity and short circuited at theopposite end of the cavity, the inner conductor of the second coaxialline entering at the other end of the cylindrical cavity and extendingbetween and parallel to the parts of the first conductor andelectrically coupled to said first conductor between its ends withinsaid cylindrical cavity and said third and fourth inner conductors beingelectrically coupled to the external side of said spaced parts of saidfirst inner conductor at equal distances from the short circuited end ofsaid first inner conductor.

2. A four terminal network comprising four coaxial transmission lines,the outer conductors of the four lines being electrically connected to acylindrical shell forming a cylindrical cavity, the inner conductor ofthe first coaxial line entering through one end of the cylindricalcavity and divided into two substantially spaced parallel parts of thesame conductor providing a pair of substantially opposed slots extendingsubstantially the length of the cavity and said parts being electricallyconnected at the opposite end of the cavity, the inner conductor of thesecond coaxial line entering at the other end of the cylindrical cavityand extending between and parallel to the parts of the first innerconductor and connected to said first inner conductor between its endswithin said cylindrical cavity and said inner conductors of the thirdand fourth coaxial line being connected to diametrically opposed sidesof the first inner conductor.

3. A four terminal network comprising four coaxial transmission lines,the outer conductors of the four lines being electrically connected to acylindrical shell forming a cylindrical cavity, the inner conductor ofthe first line entering through one end of the cylindrical cavity andforming a hollow cylindrical conductor with slots on opposite sides ofthe conductor extending substantially for the length of the conductorwithin the cavity and said inner conductor being electrically connectedto the 0pposits end of the cavity, the inner conductor of the secondcoaxial line entering at the other end of the cylindrical cavity andextendin between and parallel to the parts of the first inner conductorand connected to said first inner conductor between its ends to a sideof the inner conductor between said slots and the inner conductor ofsaid third and fourth coaxial lines being connected to diametricallopposed sides of said first inner conductor at equal distances from saidopposite end of said cavity and the end of the slot most remote fromsaid opposite end.

4. A four terminal network comprising four coaxial transmission lines,the outer conductors of the four lines being electrically connected to acylindrical shell forming a cylindrical cavity, the inner conductor ofsaid first coaxial line entering through one end of the cylindricalcavity and dividing into two substantially spaced parallel parts of thesame conductor for the length of the cavity and being short circuited atthe opposite end of said cavity and connected to said shell, the innerconductor of the second coaxial line entering at the other end of thecylindrical cavity and extending between and parallel to the spacedparts of the same first inner conductor and connected to said firstinner conductor between its ends within the cylindrical cavity, thedistance between said short circuited end and the point of connection ofsaid second inner conductor to the first being an odd multiple of aquarter wave length of the frequency at which said network is to beoperated, and said third and fourth innor conductors being electricallyconnected to the diametrically opposed sides of said first innerconductor at equal distances from the short circuited end of said firstinner conductor.

5. A four terminal network comprising four coaxial transmission lines,the outer conductors of the four lines being electrically connected to acylindrical shell forming a cylindrical cavity, the inner conductor ofthe first coaxial line entering through one end of the cylindricalcavity and divided into two substantially spaced parallel parts of thesame conductor for the length of the cavity and short circuited at theopposite end of the cavity and connected to said shell at said shortcircuited end, the inner conductor of the second coaxial line enteringat the other end of the cylindrical cavity and extending between andparallel to the spaced parts of the same first inner conductor andconnected to said first inner conductor substantially mid-way betweenits ends within said cylindrical cavity and said third and fourth innerconductors being electrically connected to the diametrically opposedsides of the first inner conductor at equal distances from the shortcircuited end of said first inner conductor.

6. An arrangement as set forth in claim 5 in which the connection ofsaid second inner conductor to the first inner conductor is at a pointsubstantially onequarter wave length of the operating frequency of saidnetwork from the point of entrance of the second 'conductor into thecavity.

7. An arrangement as set forth in claim 5 in which said third and fourthinner conductors are connected References Cited in the file of thispatent UNITED STATES PATENTS 2,454,907 Brown Nov. 30, 1948 2,465,245Mabry Mar. 22, 1949 2,480,182 Clapp Aug. 30, 1949 2,541,009 TashjianFeb. 6, 1951 OTHER REFERENCES King: Two Simple Bridges for VHF Use,Proceedings of the I. R. B, vol. 38, No. 1, January 1950, pp. 3739. Copyin 171-9543.

